1. Field of the Invention
The invention relates to plasticizer-containing films based on polyvinyl acetal with UV protection and low yellow value and to glass laminates produced therewith.
2. Background Art
Commercially available plasticizer-containing polyvinyl butyral film (PVB film) for laminated glazings has been provided with UV absorbers for many decades so that UV radiation cannot penetrate through the laminated glass completely, in contrast to visible light. Protection against harmful UV radiation for individuals or objects located behind the laminated glass can thus be ensured in a simple manner. Besides the protection against cuts, this constitutes the main property advantage compared with monolithic glass, since this is largely transparent to UV radiation.
Derivatives of 2-hydroxyphenyl benzotriazole have proven their worth and are used in a widespread manner as UV absorbers and are obtainable using different substitution patterns, for example under the trade names Tinuvin P, Tinuvin 328, Tinuvin 327 and Tinuvin 326. The benzotriazole type UV absorbers mentioned here are known in PVB films for their excellent long-term stability, good solubility and high absorption capacity over almost the entire UV range. Furthermore, the specified UV absorbers can be introduced particularly easily into the film composition during the extrusion process since they are also relatively well soluble even in the plasticizers used conventionally, such as 3G8, and can thus be easily added to the plasticizer before extrusion. In addition, the above-mentioned UV absorbers have melting points that lie significantly below the extrusion temperature approximately 200° C. conventional with PVB film. These solubility and melting properties of the specified UV absorbers are promoted by the relatively low molecular weight thereof. This, together with the good solubility in the film matrix, also leads however to a high migration ability therein. Particularly in the field of architecture, laminated safety glass (LSG), once installed, is only replaced in some circumstances after a number of decades, and therefore the question is raised as to whether permanent UV protection can be maintained with creeping loss of UV absorber via the open edges of the LSG.
In addition, properties that are harmful to health or the environment have been attributed to some of the aforementioned benzotriazole UV absorbers. Accordingly, it is undesirable if, when wiping down LSG elements (for example as part of a glass counter on which food is served), even just small quantities of UV absorber are dispensed via the edges with each wiping process and are then distributed over the surface.
Depending on the substitution pattern of the specified UV absorbers, absorption also takes place in the visible range of the spectrum to a greater or lesser extent in addition to absorption in the UV range, such that these UV absorbers cause a certain yellowing of the PVB film depending on the quantity used and substituent distribution. Here, it has been found that, with identical quantities, the influence on yellowing of the film decreases among Tinuvin 326, Tinuvin 327, Tinuvin P and Tinuvin 328. Nevertheless, there is a need for alternative UV absorbers, which, with a UV filter effect equal to or better than UVA 328, contribute even less to yellowing.
Besides UV absorbers, polyvinyl butyral is often provided with phenolic antioxidants, of which the purpose is to inhibit radical cleavage reactions triggered by high processing temperatures during film extrusion. The length of the polymer chains is thus kept substantially constant, so that the mechanical properties of the extruded material are not impaired as a result of the extrusion process.
Many phenolic antioxidants are associated however with the disadvantage of also causing yellowing, depending on the extrusion conditions, which can be attributed to reaction products of the antioxidants. Even in a finished PVB film already adhesively bonded between two glass panes, phenolic antioxidants can cause subsequent yellowing under UV irradiation, in particular with long-term increased temperature, which is not only undesirable for aesthetic reasons, but also reduces the light transmission of the glass laminate over time.
The most commercially significant group of phenolic antioxidants for the production of PVB film contains the partial structure (3,5-di-tert-butyl-4-hydroxyphenyl)propionate and leads to yellowing with use in PVB film with UV and/or temperature influence in the glass laminate. Compounds having this partial structure are offered under the trade names Irganox 1010, Irganox 1076, Irganox 1035 and Hostanox O 3.